WO2004045066A1

WO2004045066A1 - Method and device for training an rf amplifier linearization device, and mobile terminal incorporating same

Info

Publication number: WO2004045066A1
Application number: PCT/FR2003/003204
Authority: WO
Inventors: Sandrine Touchais; Xavier Checoury
Original assignee: Eads Telecom
Priority date: 2002-11-05
Filing date: 2003-10-28
Publication date: 2004-05-27
Also published as: FR2846813B1; EP1559192A1; WO2004045066A8; FR2846813A1; AU2003295008A1; ATE319222T1; DE60303819T2; US20060013334A1; EP1559192B1; DE60303819D1; ES2259153T3; US7680209B2; CA2504618A1

Abstract

The invention concerns a method whereby a linearization training sequence designed to be transmitted by means of a radiofrequency transmitter incorporated in a mobile terminal or a base station of a radiocommunication system comprising a fixed network and which is adapted for transmitting bursts in accordance with a specific frame structure, is included in a sequence of symbols (55) further designed to enable the transmission chain parameters to be adjusted.

Description

METHOD AND DEVICE FOR LEARNING A LINEARIZATION DEVICE OF AN RF AMPLIFIER, AND MOBILE TERMINAL INCORPORATING SUCH A DEVICE

The present invention relates to the linearization of radio frequency (RF) power amplifiers. It finds applications, in particular, in the RF transmitters of mobile terminals of digital radiocommunication systems. It can also be applied in RF transmitters of base stations, in particular when this station is started for the first time.

In current digital radiocommunication systems, it is sought to transmit information with a maximum bit rate in a given RF frequency band which is assigned to a transmission channel (hereinafter radio channel). To do this, the modulations used for a few years include a phase or frequency modulation component and an amplitude modulation component.

In addition, radio channels coexist in a specific frequency band allocated to the system. Each radio channel is subdivided into logical channels by time division. In each time slot ("Time Slot" in English), a group of symbols is called a burst or packet ("Burst" in English).

It is necessary to ensure that, at all times, the power level transmitted in each radio channel does not interfere with communications in an adjacent radio channel. Thus, specifications require that the power level of an RF signal transmitted in a determined radio channel is, in an adjacent radio channel, for example 60 dB (decibels) lower than the power level of the RF signal transmitted in said channel radio determined.

It is therefore necessary that the spectrum of the signal to be transmitted, which results in particular from the type of modulation used and the bit rate, is not distorted by the RF transmitter. In particular, the RF transmitter must have a characteristic of the output power as a function of the input power, which is linear. However, the radio frequency power amplifier (hereinafter RF amplifier) present in the RF transmitter has a linear characteristic with low output power but non-linear as soon as the power exceeds a certain threshold. We also know that the efficiency of the RF amplifier is all the better when we work in an area close to saturation, that is to say in non-linear regime. Thus the need for linearity and the need for high efficiency (to save the battery charge) oblige to use linearization techniques to correct the non-linearities of the RF amplifier. Two of the most commonly used techniques are adaptive baseband predistortion and the Cartesian baseband loop.

In the Cartesian loop technique, the signal to be transmitted is generated in baseband in I and Q format. Furthermore, a coupler followed by a demodulator makes it possible to take part of the RF signal transmitted and to transpose it into band of base (downconversion), in I and Q format. This baseband signal is compared to the baseband signal to be transmitted. An error signal resulting from this comparison attacks a modulator, which ensures the transposition towards the field of radio frequencies (uplink conversion). The signal at the output of the modulator is amplified by an RF amplifier which delivers the transmitted RF signal. In the adaptive baseband predistortion technique, the signal to be transmitted is generated in baseband, in I and Q format, and pre-distorted via a predistortion device. Then, this signal is transposed to the RF domain using an RF modulator. Then it is amplified in an RF amplifier. A coupler followed by an RF demodulator makes it possible to take part of the transmitted RF signal and to transpose it into baseband, in I, Q format. This demodulated signal in baseband is digitized and compared with the signal in baseband. to emit. An adaptation of the predistortion coefficients, carried out during a learning phase of the predistortion device, makes it possible to converge the signal in demodulated I and Q format towards the signal in I and Q format to be transmitted.

In these two techniques, part of the signal transmitted is taken at the output of the RF amplifier in order to compare it with the signal to be transmitted. The result that the linearity is not obtained immediately but only after a certain time, necessary for the convergence of the linearization device. Learning the linearization device requires the transmission of a particular data sequence or learning sequence. This remark certainly applies more to adaptive predistortion than to the Cartesian loop, even if the latter requires, to ensure its stability, initial adjustments of phase and amplitude levels comparable to learning.

The learning method disclosed in document WO 94/10765, is thus based on the transmission by the transmitters of the system of particular sequences, called linearization learning sequences, during linearization learning phases. More particularly, training sequences are transmitted in isolation in time intervals forming a particular logical channel of the radio channels, which is dedicated only to linearization. However, this method has several drawbacks. First of all, it requires prior synchronization of all the transmitters so that they transmit their respective linearization learning sequence in the logic channel dedicated to linearization. In addition, no data transmission can take place in the time slots of this logical channel. In addition, at the start of each transmission or in the event of a change of radio channel, the transmitter is obliged to wait for the next time interval of the logical channel dedicated to linearization, unless the system becomes considerably more complex. This is why the temporal spacing between two time intervals of said logical channel cannot exceed the second, in order to guarantee a certain quality of service (QoS). This technique is therefore very detrimental to the spectral efficiency of the radiocommunication system.

In general, there are radiocommunication systems whose frame structure is not suitable for the transmission of a training sequence, for example when no specific time interval has been provided for this purpose during of the definition of the frame structure. In order to overcome all or part of the drawbacks of the aforementioned prior art, a first aspect of the invention relates to a method for learning a device for linearizing a radiofrequency amplifier which is included in a radiofrequency transmitter of a terminal. mobile of a radiocommunication system comprising a fixed network and mobile terminals, which transmitter is adapted to transmit bursts according to a determined frame structure, each burst comprising symbols belonging to a determined symbol alphabet. The method comprises the steps of: a) generating a linearization training sequence comprising a determined number N of symbols, where N is a determined integer; b) transmitting the linearization training sequence by means of the radiofrequency transmitter, in at least some of the bursts emitted by the latter; c) comparing the linearization training sequence sent to the linearization training sequence generated in order to train said linearization device.

Advantageously, in step b), the linearization learning sequence is included in a sequence of symbols further provided for allowing the adjustment of parameters of the transmission chain between said first equipment and a second equipment of the radiocommunication system with which said first equipment communicates.

By transmission chain is meant all of the components which take part in bidirectional communication between first and second equipment, typically a mobile terminal and the base station with which it communicates.

Preferably, the sequence of symbols provided to allow the adjustment of parameters is a sequence of symbols provided to allow the dynamic control of the gain of a variable gain amplifier of a radiofrequency receiver of a second item of equipment of the radiocommunication system with which the first device communicates. In other words, the training sequence is transmitted in step b) within a time interval reserved in the frame structure for the transmission of an AGC (automatic gain control) sequence, and at the same time it fulfills the role of such a CAG sequence. Thus, the transmission time of a sequence of symbols necessary for other purposes is used for the transmission of the learning sequence, in this case a CAG sequence transmitted to allow dynamic control of the power d transmission of the mobile terminal on reception.

According to one advantage, the value of the symbols of the AGC sequence is not subject to any constraint (the AGC sequence must simply be known to the fixed network). There is therefore complete freedom to choose the symbols of the sequence, or at least part of the symbols of the sequence, so that these symbols form a satisfactory learning sequence.

According to another advantage, the recurrence of the AGC sequence is adapted to the needs of learning the linearization device of the RF amplifier. In fact, the AGC sequence is generally transmitted at the start of the frame, then during a change of logical channel, during a change of RF frequency and / or during a change of power level. However, it is also at these moments that the linearization learning sequence needs to be transmitted.

A second aspect of the invention relates to a device for learning a device for linearization of a radiofrequency amplifier which is included in a radiofrequency transmitter of a first item of equipment of a radiocommunication system, which transmitter is adapted for transmitting bursts according to a determined frame structure, each burst comprising symbols belonging to a determined symbol alphabet. The device comprises: a) means for generating a linearization training sequence comprising a determined number N of symbols, where N is a determined integer; b) means for transmitting the linearization training sequence by means of the transmitter in at least some of the bursts emitted by the latter; c) means for comparing the linearization training sequence transmitted with the linearization training sequence generated in order to drive said linearization device.

Advantageously, the linearization learning sequence is included in a sequence of symbols further provided for allowing the adjustment of parameters of the transmission chain between said first equipment and a second equipment of the radiocommunication system with which said first equipment communicates.

Preferably, the sequence of symbols provided to allow the adjustment of parameters is a sequence of symbols provided to allow the dynamic control of the gain of a variable gain amplifier of a radiofrequency receiver of a second item of equipment of the radiocommunication system with which the first device communicates.

In other words, said means for transmitting are adapted to transmit the training sequence within a time interval reserved in the frame structure for the transmission of a CAG sequence, and the training sequence ensures at the same time the role of such a CAG sequence.

A third aspect of the invention relates to a mobile terminal of a radiocommunication system, comprising a radiofrequency transmitter

having a radiofrequency amplifier and a device for linearizing the radiofrequency amplifier, which further comprises a device for learning the linearization device according to the second aspect.

A fourth aspect of the invention relates to a base station of a radiocommunication system, comprising a radiofrequency transmitter having a radiofrequency amplifier and a device for linearizing the radiofrequency amplifier, which further comprises a device for learning the radiofrequency device. linearization according to the third aspect. Other characteristics and advantages of the invention will become apparent on reading the description which follows. This is purely illustrative and should be read in conjunction with the accompanying drawings in which:

- Figure 1 is a block diagram of an example of mobile terminal according to the invention;

- Figure 2 is a diagram illustrating a first example of bursts emitted by the mobile terminal, without CAG sequence;

FIG. 3 is a diagram illustrating a second example of bursts emitted by the mobile terminal, with a CAG sequence which according to the invention comprises a linearization training sequence; and,

- Figure 4 is a diagram illustrating the implementation of an AGC process between first and second equipment, and vice versa.

In Figure 1, there is shown schematically the means of an example of a mobile terminal according to the invention. Such a mobile terminal belongs, for example, to a radiocommunication system which also comprises a fixed network having base stations.

The terminal comprises a transmission chain 100, a reception chain 200, a control unit 300, as well as a permanent memory 400, as well as an automatic gain control device (AGC) associated with an RF receiver of the reception chain 200.

The transmission chain 100 comprises a useful data source 10, for example a speech coder delivering data encoding voice. The source 10 is coupled to an M-ary data modulator 20 which provides baseband modulation of the data to be transmitted according to a modulation with M distinct states, where M is a determined integer. The binary data it receives from the source 10 are translated by the modulator 20 into symbols belonging to an M-ary alphabet, that is to say comprising M distinct symbols. The output of the modulator 20 is coupled to the input of a radiofrequency transmitter 30. From the series of symbols received, the transmitter 30 produces an RF signal suitable for radio transmission via an antenna or a cable. The output of the transmitter 30 is coupled to an antenna transmission / reception 40 via a switch 41. Thus the RF signal produced by the transmitter is transmitted on the radio channel associated with the transmitter.

The reception chain 200 comprises a radio frequency receiver 50 which is coupled to the antenna 40 via the switch 41, to receive an RF signal. The receiver 50 transposes the RF domain to the baseband (downward conversion). The reception chain 200 also includes an M-ary data demodulator 60, coupled to the receiver 50. The data demodulator 60 provides in baseband the demodulation of the data of the received signal, that is to say the reverse operation of that provided by the modulator 20. Finally, the reception chain 200 comprises a data consumer device 70, such as a speech decoder, which is coupled to the demodulator 60. This device receives as input the binary data delivered by the demodulator 60 .

The unit 300 is for example a microprocessor or a microcontroller which manages the mobile terminal. In particular, it controls the data modulator 20, the data demodulator 60, the transmitter 30 and the switch 41. It also generates signaling data which is supplied to the modulator 20 to be transmitted in appropriate signaling logical channels. Conversely, the unit 300 receives from the data demodulator 60 signaling data sent by the fixed network in appropriate logical signaling channels, in particular synchronization information and operating commands.

The memory 400, is for example a ROM memory (“Read Only

Memory ”), EPROM (“ Electrically Programable ROM ”) or Flash-EPROM, in which data is stored which is used for the operation of the mobile terminal. These data include in particular a linearization training sequence to which we will return later.

An embodiment of the transmitter 30 will now be described in detail. In this example, the transmitter 30 comprises a radiofrequency power amplifier 31, a radiofrequency modulator 32 which transposes the baseband to the radiofrequency domain ( conversion up), a linearization device 33, a learning module 34 associated with the linearization device.

The output of the power amplifier 31 delivers the RF signal to be transmitted. This is why it is coupled to the antenna 40 via the switch 41. The input of the power amplifier 31 receives a radiofrequency signal delivered by the output of the radiofrequency modulator 32. The input of the latter is coupled to the output of the data modulator 20 to receive the series of symbols forming the baseband signal to be transmitted, through the linearization device 33. The latter includes for example a predistortion device comprising a palette ("look-up table") which translates each value of the signal to be transmitted into a pre-distorted value. As a variant or in addition, the device 33 may also include means for controlling the amplitude of the signal at the output of the transmitter 30.

The learning module 34 teaches the linearization device 33 as a function of an input signal which reflects the RF signal delivered by the output of the power amplifier 31. To this end, the module 34 receives a part of this RF signal, which is taken at the output of the power amplifier 31 by means of a coupler 36. As necessary, the module 34 ensures the return to baseband of the RF signal thus taken. Although being shown entirely inside the transmitter 30, the module 34 can, at least in part, be implemented by means belonging to the control unit 300, in particular software means.

Finally, the automatic gain control device 500 allows the control unit 300 to dynamically vary the gain of the variable gain amplifier 59 of the RF receiver 50, as a function of information which is received from the base station. with which the terminal communicates, according to a process known in itself. By virtue of this method, the base station transmits at determined times a determined sequence, called the AGC sequence. This sequence is known to and recognizable by the mobile terminal. It allows it to measure the power of the signal received from the base station and to deduce therefrom a gain control of the amplifier 59. This method is implemented in the mobile terminal by the device 500 under the control of the unit 300.

The principle of such a method will be described below with reference to the diagram in FIG. 4. We will now describe the operation of the mobile terminal during a learning phase, by the device 34, of the linearization device 33. Although this is not mentioned each time in the following, it is understood that the terms "learning phase" and the terms "learning sequence" refer to the learning of the linearization device 33 carried out by the device d learning 34 under the control of unit 300.

The learning method of the device 33 comprises a step consisting in generating a learning sequence comprising a determined number N of symbols, where N is an integer. This step is carried out by the data modulator 20 under the control of the control unit 300. To this end, the unit 300 reads a corresponding sequence of bits in the memory 400.

Then, still under the control of the unit 300, the learning sequence is transmitted by means of the transmitter 30 in at least some of the bursts transmitted by the latter, according to the frame structure of the system.

The learning device 34 then obtains the transmitted learning sequence and compares it to the generated learning sequence, and consequently performs actions such as adaptations of predistortion coefficients or others of the linearization device 33, according to an algorithm determined learning. This algorithm can be adaptive. We speak of training to designate these operations.

It can be noted that for any modulation, it is possible to find a signal sequence of determined length N whose characteristics meet constraints imposed in terms of spectral width, depth of amplitude modulation, and / or others. In an example, N is equal to 10. The AGC sequence includes at least N symbols. It can therefore have a length greater than that of the training sequence, when it includes more than N symbols. In this case, the symbols of the training sequence are preferably the symbols of the AGC sequence which are transmitted first. In this way, the convergence of the learning algorithm and therefore the linearization of the RF amplifier are obtained as quickly as possible.

Learning phases can be carried out periodically or otherwise. Other constraints may have to be taken into account after the initial learning phase, when it is just appropriate to correct drifts in the transmitter. The learning sequence can therefore evolve both in content and in length. The number N is therefore not necessarily fixed from one transmission of the learning sequence to another. If an increase in the size of the sequence poses problems (for example if the frame structure is not very flexible), one can fix the size N of the sequence and just modify its content according to the evolution of the constraints on the system. .

The diagram in Figure 2 illustrates a first example of a burst, which does not include a CAG sequence. In this example, the burst has a duration equal to 20 ms. It firstly comprises a ramp up 51 ("ramping-up" in English) of 625 μs, comprising five stuffing symbols, to ensure the ramp-up. By padding symbols, it is meant that the binary data transmitted in this climb ramp are padding bits, that is to say, for example, a sequence of 0s. It then comprises a sequence of synchronization data 52 whose duration is approximately 5 ms. Next, it comprises a useful data sequence 53. The useful data can be voice coding data and more generally traffic data, or signaling data depending on whether the burst is emitted on a logical traffic channel or a channel. signaling logic, respectively. Finally, it includes a descent ramp 54, again having five stuffing symbols for the descent in power. Optionally, a guard time is also provided after the issuance of a burst, in order to guarantee the return to reception of the transmitter. In addition, in any frame structure it is intended to send isolated bursts, in particular at each change of logical channel (occurring in particular at each reversal, that is to say passage from a reception phase to a phase transmission frequency), at each RF frequency change (when a frequency hopping functionality is implemented by the system), at each change in transmission power level, or in other specific cases that it would take too long to detail here.

FIG. 3 shows an example of such an isolated frame comprising, before the synchronization sequence 52, a CAG sequence referenced 55. This sequence 55 is transmitted to allow dynamic control, by the fixed network, of the transmission power of the 'transmitter (see above). In this example, sequence 52 and sequence 55 last only 1 to 3 ms each. The other parts of the burst are unchanged with respect to the burst in FIG. 2. The useful data sequence 53 can however be shorter than in the case of a normal burst according to FIG. 2.

Advantageously, a part of these isolated bursts is used to allow the learning device 34 of the radiofrequency transmitter 32 to execute a learning algorithm of the linearization device 33. In the example of FIG. 3, the sequence of linearization is thus included in the aforementioned CAG sequence.

It is thus possible to use the time necessary for the transmission of the learning sequence for other purposes such as for example the adjustment of the AGC on reception, according to the method which was mentioned above with regard to the diagram of Figure 1.

The AGC sequence, and therefore the learning sequence, are preferably sent at the start of the frame, then during a logic channel change, during an RF frequency change and / or during a level change of power and / or in other cases that it would be too long to detail here. This is why it is particularly advantageous to combine these sequences (the training sequence being included in the CAG sequence). According to another advantage, the AGC sequence is located as close as possible to the ramp-up of the signal, for example, just following this ramp. In this way, the linearization device can be learned as quickly as possible and thus disturb the transmission for the shortest possible time.

It is preferable that the length of the training sequence is such that it does not occupy too large a portion of the burst in order to keep a maximum of symbols for the dissemination of useful information. This duration obviously depends on the precision sought for the learning algorithm but a compromise between precision and duration is often necessary in order to keep a maximum of useful information in the burst. A reasonable compromise is reached when it represents approximately 5% of the total duration of the salvo. In the case of a 20 ms burst transmitted at a binary rate of 8 ksymbols / s, the duration of a learning sequence of N = 10 symbols is thus equal to 1.25 ms or 6.25% of the total frame time.

The diagram in FIG. 4 illustrates the implementation of an AGC method (known per se) between a first device 5 and a second device 5 ′ of a radiocommunication system.

The equipment 5 is here a mobile terminal for example as described above with reference to FIG. 1. It comprises the RF transmitter 30 and the RF receiver 50, the latter comprising the variable gain amplifier 59. The equipment 5 'is here a base station with which the mobile terminal 5 communicates, which comprises an RF transmitter 30' and an RF receiver 50 'having a variable gain amplifier 59'. Functionally, the components 30 ', 50' and 59 'of the base station 5' are identical or comparable to the components 30, 50 and 59 respectively of the mobile terminal 5 '. These components are not detailed again here.

A sequence of AGCs transmitted by the mobile terminal 5 allows dynamic control of the gain of the amplifier 59 'of the receiver 50' of the base station 5 '. Conversely, a sequence of AGCs transmitted by the 5 'base station allows the dynamic gain control of the amplifier

59 of the receiver 50 of the mobile terminal.

Claims

1. Method for learning a device for linearizing a radiofrequency amplifier (31) which is included in a radiofrequency transmitter (30) of a first item of equipment (5) of a radiocommunication system, which transmitter is suitable for send bursts according to a determined frame structure, each burst comprising symbols belonging to a determined symbol alphabet, the method comprising the steps consisting in: a) generating a linearization training sequence (FIG. 3) comprising a determined number N symbols, where N is a determined whole number; b) transmitting the linearization training sequence by means of the transmitter in at least some of the bursts emitted by the latter; c) comparing the linearization training sequence transmitted with the linearization training sequence generated in order to train said linearization device, characterized in that, in step b), the linearization training sequence is included in a sequence of symbols further provided for adjusting parameters of the transmission chain between said first equipment and a second equipment (5 ') of the radiocommunication system with which said first equipment communicates.

2. Method according to claim 1, according to which the sequence of symbols intended to allow the adjustment of parameters is a sequence of symbols provided to allow the dynamic control of the gain of a variable gain amplifier of a radio frequency receiver of a second equipment of the radiocommunication system with which the first equipment communicates.

3. Method according to claim 1 or claim 2, according to which the linearization training sequence occupies only a part of the burst in which it is emitted.

4. Method according to claim 3, according to which the linearization training sequence occupies approximately 5% of the duration of the burst in which it is emitted.

5. Method according to any one of the preceding claims, according to which the linearization training sequence is transmitted at the start of the frame.

6. Method according to any one of the preceding claims, according to which the linearization training sequence is further transmitted during a change of logic channel, a change of frequency and / or a change of level of power of the first equipment.

7. Method according to any one of the preceding claims, according to which the sequence of symbols provided for enabling the dynamic control of the transmission power of the mobile terminal comprises more than N symbols, and according to which said N symbols of the sequence of linearization learning are the symbols of the symbol sequence intended to allow dynamic control of the transmit power of the mobile terminal which are transmitted first.

8. Learning device for a linearization device (33) of a radiofrequency amplifier (31) which is included in a radiofrequency transmitter (30) of a first equipment of a radiocommunication system, which transmitter is adapted for send bursts according to a determined frame structure, each burst comprising symbols belonging to a determined symbol alphabet, the device comprising: a) means (300,10,20) for generating a linearization training sequence comprising a number determined N of symbols, where N is a determined integer; b) means (300,30) for transmitting the linearization training sequence by means of the transmitter in at least some of the bursts emitted by the latter; c) means (300,34) for comparing the linearization training sequence transmitted with the linearization training sequence generated in order to drive said linearization device, characterized in that said linearization training sequence is included in a sequence of symbols further provided for adjusting parameters of the transmission chain between said first equipment and a second equipment (5 ') of the radiocommunication system with which said first equipment communicates.

9. Device according to claim 8, in which the sequence of symbols intended to allow the adjustment of parameters is a sequence of symbols provided to allow the dynamic control of the gain of a variable gain amplifier of a radio frequency receiver of a second equipment of the radiocommunication system with which the first equipment communicates.

10. Device according to claim 8 or claim 9, in which the linearization training sequence occupies only a part of the burst in which it is emitted.

11. Device according to claim 10, in which the linearization training sequence occupies approximately 5% of the duration of the burst in which it is emitted.

12. Device according to any one of claims 8 to 11, wherein said means for transmitting are adapted to transmit the linearization training sequence at the start of the frame.

13. Device according to any one of claims 8 to 12, in which said means for transmitting are adapted to further transmit the linearization training sequence during a change of logical channel, a change of frequency and / or a change in power level of the mobile terminal.

14. Device according to any one of claims 8 to 13, in which the sequence of symbols provided to allow the adjustment of parameters comprises more than N symbols, and in which said N symbols of the linearization training sequence are the symbols of the symbol sequence intended to allow the adjustment of parameters which are transmitted first.

15. Mobile terminal (5) of a radiocommunication system, comprising a radiofrequency transmitter (30) having a radiofrequency amplifier and a device for linearizing (33) the radiofrequency amplifier, characterized in that it further comprises a device learning the linearization device according to any one of claims 8 to 14.

16. Base station (5 ') of a radiocommunication system, comprising a radiofrequency transmitter (30') having a radiofrequency amplifier and a device for linearizing the radiofrequency amplifier, characterized in that it further comprises a device learning the linearization device according to any one of claims 8 to 14.